In general, various panels such as floor warming panel, etc. have so far been manufactured by injecting liquid of a heat insulating material such as urethane foam, etc. into an outer shell structure interior in a hollow planar shape with conduit tube for passing heating medium through it being arranged, thereby forming an integral heat insulating layer. However, in injecting liquid of the heat insulating material such as urethane foam, etc. into the outer shell structure, the liquid running at a high speed collides with the outer shell structure, which tends to cause displacement of those fixed parts such as stud nuts which have been tentatively secured at positions inside the outer shell structure, where they meet the gushing liquid of the insulating material, or breakage and damage to those joined portions at such colliding positions with the consequent high rate of yield of disqualified product.
Similarly, in the injection into the outer shell structure of the liquid such as urethane foam, etc., the liquid running at a high speed collides with the inner wall of the outer shell structure opposite to the injection port from the direction substantially perpendicular to it, on account of which the liquid in the outer shell structure takes a random flowing direction to cause, in most cases, unfilled portion, i.e., mold cavity to be created at the center part of the outer shell structure which is structurally unstable with the consequent high rate of yield of disqualified product.